Though he didn't say so, when he was governor Massachusetts passed legislation that required residents to purchase coverage - the so-called individual mandate that conservatives and he oppose on a national level.

Romney also said that Obamacare would cut $716 billion from Medicare over the next decade.

The president said the changes were part of a plan to lengthen the program's life, and he added that AARP, the seniors lobby, supports it.

With a two-minute closing statement, Obama said he had spent his first four years in office fighting for those in the middle class and those seeking to make it there. "If you'll vote for me, I'll fight just as hard in my second term," he said.

Romney was as critical of Obama's tenure as he was the moment the two men walked onto the stage.

If the president is re-elected, he predicted continued economic trouble for the middle class, chronic unemployment, higher costs for health insurance and "dramatic cuts to the military."

Obama took office in the shadow of an economic crisis but promised a turnaround that hasn't materialized. Economic growth has been sluggish throughout his term, with unemployment above 8 percent since before he took office.

The two presidential rivals also are scheduled to debate on Oct. 16 in Hempstead, N.Y., and Oct. 22 in Boca Raton, Fla.

Vice President Joe Biden and Republican Rep. Paul Ryan of Wisconsin have one debate, Oct. 11 in Danville, Ky. Both men have already begun holding practice sessions.